The present invention relates to an image forming apparatus having a photoreceptor on which an electrostatic latent image is formed by exposure, and more particularly to an image forming apparatus that has been improved so as to control the electric potential of the photoreceptor to be uniform before the photoreceptor is exposed.
There are known image forming apparatus having a photoreceptor such as an electrophotographic copier, laser printer, electrophotographic facsimile and the like.
In an electrophotographic copier (hereinafter referred to as a copier) for example, there is provided a photoreceptor of a drum shape (hereinafter referred to as a photoreceptor drum). A photoreceptor drum is generally made of an aluminum tube and provided with a photosensitive semiconductor layer at the surface thereof The above copier is designed such that during image formation, the photosensitive semiconductor layer at the surface of the photoreceptor drum is charged to have a uniform potential by corona discharge of a charger for charging the surface of the photoreceptor drum, and the photosensitive semiconductor layer thus charged is then exposed to light thereby eliminating the electric charge in the exposed area thereof which will form an electrostatic latent image.
However, it often occurs that the electric charges are not evenly distributed in the photosensitive semiconductOr layer. It is required for obtaining an electrostatic latent image of good quality to control the distribution of electric charges.
To solve the above problem, such a method has been conventionally adopted that the condition of the electric charge distribution in a photosensitive semiconductor layer is checked by a measuring instrument in a manufacturing process of a copier and if the electric charges are not evenly distributed, they are adjusted.
Another method that has been adopted is that a potential sensor for measuring a potential at the surface of a photoreceptor drum is disposed in the vicinity of the photoreceptor drum and the measured value obtained by the potential sensor is fed back to a charger-driving circuit thereby maintaining a uniform potential at the surface of the photoreceptor drum.
In the case where a potential sensor is employed, a correction is required in the output from the potential sensor for the two reasons described below.
(a) The sensitivity varies depending on a potential sensor; and
(b) When installing a potential sensor oppositely to the surface of a photoreceptor drum, it is a particular problem that the distance between the surface of a drum and a potential sensor slightly varies depending on a product.
To correct the output from the potential sensor, such a method is convenient that a high voltage is directly applied to the photoreceptor drum, the output from the potential sensor at that time is read out, and then a correction value is determined from the relationship between the output from the potential sensor and the high voltage that has been applied to the drum.
In a conventional copier, as illustrated in FIG. 11, a photoreceptor drum 1 is disposed being earthed via a connecting wire 2, connector 3 and connecting wire 4, and a developing roller 5 is connected to a developing bias circuit 9 via a connecting wire 6 and connector 7 and connecting wire 8. With use of the circuit having the above arrangement, the connector 7 is manually exchanged with the connector 3, a high voltage is applied to the photoreceptor drum 1 by means of the developing bias circuit 9, and then the output from the potential sensor 10 is read out at that time thereby correcting the output from the potential sensor.
The above method, however, can be only adopted on limited occasions, in a manufacturing process of a copier or at the time of maintenance carried out by a customer service engineer, for example.
Furthermore, the potential sensor is apt to become dirty with toner or the like during its use and this gradually deteriorates the sensitivity of the potential sensor. Therefore, a correction is also required in the output from the potential sensor in the above case and reproductions of a good quality will not be obtained unless a correction is made.
Not only copiers but also image forming apparatus of all types including laser printers and electrophotographic facsimiles and the like have the same problem as the above-mentioned.
The object of the present invention is to provide an image forming apparatus in which the output from the potential sensor can be automatically corrected during the use of a copier.